This invention relates to a circuit for adaptively adjusting the operating point of a transistor in saturation to maintain the forced current gain of the transistor within a desired range throughout a desired range of load conditions and operating temperatures. More particularly, this invention relates to an adaptive transistor drive circuit for use in association with a transistor switch subject to varying load conditions and operating temperatures.
In many applications, it is desirable to operate a transistor in a saturation condition to improve the efficiency of the transistor. One such application is the use of a transistor as a switch in a switching voltage regulator. An ideal switch is one which is capable of controlling a wide range of switch currents with no power dissipation and with an instantaneous transition from a conducting state to a non-conducting state, and vice versa.
Transistors do no fit this ideal description of a switch, but they most closely approximate an ideal switch when operated in the saturation region during the "on" condition of the switch and in the cut-off region during the "off" condition of the switch. This is because a transistor dissipates power as a function of the collector-emitter voltage (V.sub.CE) multiplied by the collector current (I.sub.C). When a transistor is operated in saturation, voltage V.sub.CE is low for a given I.sub.C. Therefore, to reduce the power dissipated by a transistor used as a switch, particularly at high collector currents, it is desirable that the transistor be operated in saturation during the "on" condition of the switch.
However, efficiency is reduced if the transistor is driven too deeply into saturation. Efficiency suffers because power is dissipated by the circuit employed to provide drive current to the transistor to drive it into saturation. When a transistor is operating in saturation, the drive current provided to the transistor exceeds the value necessary to produce the collector current I.sub.C which results. The amount of excess drive current increases, and the overall efficiency of the circuit decreases, if the transistor is driven too deep into saturation.
An additional consideration is the time required to turn the switch transistor off. Excess base current will cause long turn-off time which may interfere with circuit operation or may require extensive turn-off current drive circuitry to rapidly turn off the switch transistor.
From the foregoing discussion, it will be evident that to optimize the overall efficiency of a circuit including a transistor switch, both the power dissipated by the drive circuit and the power dissipated by the transistor must be minimized. Typically, minimum power dissipation, and optimum overall efficiency, are achieved when the transistor is operated at a point near the edge of saturation.
The point of optimum overall efficiency of a circuit including a transistor in saturation is dependent on the characteristics of the transistor, the operating temperature, and the load on the transistor, i.e., the collector current. For a predetermined value of collector current and operating temperature, the point of optimum efficiency, defined as the ratio of collector current to base current (the forced current gain) of the saturated transistor, can be determined. The circuit can be designed to provide a predetermined drive current to the switch transistor so that the switch transistor operates during "on" conditions at that optimum point, such that the power dissipated by the drive circuit and the power dissipated by the switch transistor are within desired limits. However, if the load, and therefore the switch collector current, varies, or if the temperature varies, the circuit will no longer operate with optimum efficiency if the base current remains fixed. For instance, if load current increases, but base drive current remains constant, the switch transistor will operate at a less saturated point, or drop out of saturation entirely, thus increasing the power dissipated by the circuit. Alternatively, if load current decreases, the switch transistor will be driven by excess base current deeper in saturation, and wasteful power will be dissipated by the drive circuit. Likewise, the circuit will drift away from a point of optimum efficiency with changes in operating temperature.
In view of the foregoing, it would be desirable to provide an adaptive transistor drive circuit for maintaining the operating point of a conducting transistor within a desired range of a chosen point in saturation throughout a desired range of collector currents and operating temperatures.
It would also be desirable to be able to provide an adaptive transistor drive circuit for maintaining the forced current gain of a saturated transistor within a desired range under conditions of varying collector current and operating temperature.
It would further be desirable to be able to provide an adaptive transistor drive circuit which improves the efficiency of a transistor switch operating under conditions of varying switch current and operating temperature.
It would additionally be desirable to be able to provide an adaptive transistor drive circuit for use in a monolithic integrated circuit voltage regulator, and other integrated circuits, which require an efficient switching function at varying collector currents and operating temperatures.